Post-coital warfare: insect semen kills rival sperm

Artificial insemination equipment normally used for honeybees was used to collect seminal fluid in leafcutter ants

(Image: Boris Baer, University of Western Australia)

A copulating pair of the bumblebee Bombus terrestris

(Image: Boris Baer, University of Western Australia)

If you’ve only got one shot, you better make it count. For some social insects, with only one chance to impregnate their queen, things can get nasty, but it’s not the males that try to harm each other&colon; it’s their ejaculate.

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Some female insects, such as honeybees and leafcutter ants, have sex on only one day in their life. But what a day&colon; they mate with multiple males and store enough sperm to fertilise eggs throughout their lives.

Now it seems that when honeybees and leafcutter ants inseminate the queen, their seminal fluid is harmful to rival sperm. Researchers looking at sexual selection often focus on the all-important sperm, says Boris Baer of the University of Western Australia. The seminal fluid tends to be discounted as merely a sugary liquid which provides energy, he says.

Baer and colleagues from the University of Copenhagen, Denmark, exposed the sperm of honeybees and leafcutter ants to their own seminal fluid, and the secretions of other males of the same species. The seminal fluid killed more than 50 per cent of the rival sperm within 15 minutes. “The males seemed to use the seminal fluid to harm the sperm,” says Baer.

One chance to mate

The team repeated the experiment with species that only mate once in their life such as bumblebees (Bombus terrestris) and Trachymyrmex ants. As expected the seminal fluid of these strictly monandrous species, which never encounters the sperm of a competitor, didn’t assault alien sperm.

The seminal fluid of the polyandrous species also protected and increased the survival rate of their own sperm, says Baer – which implies that like the immune system of vertebrates there is something in the seminal fluid that recognises “self” and “non-self”. “That is a weird idea,” says Baer, “that fluid is capable of doing that.”

Females also put up a fight to save the sperm inside them. Baer and colleagues found that queen leafcutter ants can chose to secrete a fluid that protects sperm from the damaging effects of seminal fluid from rival males.

Strongest sperm win

“The queens can let the warfare run as long as she wants,” says Baer, who believes the female could wait for the strongest sperm to survive the attacks before releasing her fluid to ensure she maintains enough viable sperm to fertilise all eggs needed to run a colony.

So, could we imagine a similar substance found within human seminal fluid?

“To my knowledge women do not copulate with 90 mates in half an hour, so whether there is much room that this has evolved in humans as well, I have my doubts,” says Baer. But he’s not ruling it out&colon; “In the sperm world you must be prepared for everything.

Bryan Fry, an insect biologist at the University of Melbourne, Australia, is impressed.” When it comes to the battle for insemination, Fry says, “It is always fascinating to see the techniques used by animals to reinvent the wheel.”